I got a very big problem that made my zen-cart project totally stopped.

I have installed an simplified chinese version zen cart. But then I discover that, some of the english plugins cannot be installed successfully. If I totally replace some of the files e.g. admin/includes/languages/english.php, the chinese characters turned into weird symbols. Even I follow the instruction to just addin the codes (not totally replace the file). The addon still cannot function. Does anyone know what the problem is?

Is it able to convert back this chinese zen-cart into english version? I've tried to firstly install a english zen-cart then copy all files in the includes folder back. But then the whole frontend cannot be shown anymore. Is there any proper way to do so?